1. Field of the Invention
This invention relates to the field of well logging and, more particularly, to well logging techniques and apparatus for determining formation properties, such as borehole compensated resistivity, and also for determining borehole diameter and borehole fluid resistivity with improved accuracy. The invention has general application in the well logging art, but is especially useful in measurement-while-drilling.
2. Description of the Related Art
A commonly used technique for evaluating formations surrounding an earth borehole is resistivity logging. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while porous formations with low resistivity are generally water saturated. However, the region immediately surrounding the borehole can be invaded by borehole fluid or mud filtrate and have a different resistivity than the virgin formation. If a resistivity logging device has only one radial depth of investigation, there is limited ability to measure resistivity of all zones of interest, and there may be difficulty in determining if the measured resistivity represents the invaded zone, the virgin zone, or some combination of the two zones. However, if the resistivity logging device has multiple radial depths of investigation, there is greater flexibility. In addition to the advantage of having, for example, a shallow measurement and a deep measurement individually, the combination of the two can provide additional information such as the extent of invasion. It is also possible to combine two or more measurements, for example a shallow measurement and a deeper measurement, to compute a better extimate of the true formation resistivity. Another related factor, to be discussed further below, is the effect of the borehole itself on measurements.
Wireline resistivity logging tools have long been provided with two or more radial depths of investigation. Conventional wireline resistivity logging tools typically achieve two depths of investigation by using a short and a long vertical array of electrodes or coils. In general, a long vertical array provides a greater radial depth of investigation than does a short vertical array. More recently, as will be discussed below, measurement-while-drilling tools have been provided with multiple radial depths of investigation.
A type of well logging which is of interest herein is so-called electromagnetic propagation logging, which can be used to measure the resistivity of the formation surrounding a borehole. For example, U.S. Pat. No. 3,551,797 describes a technique wherein electromagnetic energy is transmitted into the formation, and energy which returns to the borehole is measured at a receiver pair to determine the attenuation and/or the phase shift of the electromagnetic energy propagating in the formation. More than one vertical spacing between a transmitter and different receiver pairs may be used to obtain different radial depths of investigation. For example, a receiver pair relatively close to the transmitter can be used to obtain attenuation and/or phase shift information from which the properties of the invaded zone are determined, and measurements of the attenuation and/or phase shift from a receiver pair relatively far from the transmitter can be used to obtain the properties of the deeper uninvaded formations. Either attenuation or phase shift can be used to determine a bulk value of the electromagnetic skin depth for the formation, with the bulk conductivity then being determinable from the electromagnetic skin depth.
Various other techniques also exist in the art for utilizing multiple transmitters and/or receivers to investigate resistivity at different depths of investigation.
In U.S. Pat. No. 4,899,112 there is disclosed a logging apparatus for determining the resistivity of formations at two different radial depths of investigation using signals received at a single receiver pair. The resistivity of formations at a relatively shallow depth of investigation around the receiver pair is determined as a function of the phase shift measured at the receiver pair, and the resistivity of formations at a relatively deep depth of investigation around the receiver pair is determined as a function of the attenuation measured at the receiver pair. The apparatus is particularly advantageous for measurement-while-drilling, where it is desirable to obtain resistivity at multiple depths of investigation while minimizing the length and complexity of the logging device. The '112 Patent also discloses a so-called borehole compensated embodiment, where the receiver pair is located between and equally spaced from, a pair of transmitting antennas that can be alternately energized. The signals received at the receiver pair can be averaged to obtain borehole compensated signals; i.e., signals from which (1) drift of electronic components, and (2) borehole rugosity, have been reduced or removed by cancellation.
U.S. Pat. No. 4,899,112 also discloses that the phase shift imbalance (the difference in phase shift for upward and downward propagating signals) and/or the attenuation imbalance (the difference in attenuation for upward and downward propagating signals) can be used in obtaining a differential borehole caliper and, in some circumstances, a borehole caliper. The '112 Patent further indicates that a mathematical model could be used to relate the phase shift imbalance and the amplitude imbalance to the change in borehole diameter, and that, for example, the specific geometry of the tool, the size and shape of the borehole, and the properties of the mud and formation can be included in the mathematical model and in a look-up table. The '112 Patent also observes that a caliper look-up table can also be generated by performing an experiment wherein the phase shift imbalance and the attenuation imbalance are measured as a tool is moved through a borehole with a stepped diameter.
One or more additional transmitters can be added to the type of logging device described in U.S. Pat. No. 4,899,112, at different spacing(s), to attain further depths of investigation. In U.S. Pat. No. 5,594,343, assigned to the same assignee as the present application, there is disclosed a logging apparatus that can be utilized in measurement-while-drilling, and which has three or more transmitting antennas and a pair of receiving antennas. In one disclosed embodiment of the '343 Patent, there are five transmitters; that is, fifth, third, first, second, and fourth transmitting antennas longitudinally spaced on a logging device in the recited sequence. A pair of longitudinally spaced receiving antennas are located between the first and second transmitting antennas. The fifth, third, first, second, and fourth transmitting antennas are spaced from the midpoint between the receiver pair by fifth, third, first, second, and fourth distances, respectively, and each of the fifth, third, first, second, and fourth distances are different. The transmitting antennas can be individually energized, and the configuration can provide the advantages of borehole compensation without the need for having, for each transmitter to receiver pair spacing, an "equal and opposite" transmitter with the same spacing on the other side of the receiver pair.
The geometry of the borehole and the properties of the borehole fluid (also called mud herein) can be important in the determination of formation resistivity, since correction should be made for the effect of the borehole fluid on the electromagnetic energy being used to measure formation properties. The mud resistivity can generally be measured uphole with good accuracy, but the downhole mud resistivity can be affected by various factors including temperature and mixing of the mud with formation fluids. The extent of such mixing in a given zone may or may not be estimable from local geological knowledge. The borehole geometry may also be roughly approximated from the drill bit size, but can vary substantially in different types of formations.
In wireline applications it may be practical to obtain a measure of borehole geometry with a caliper tool and/or to sample or otherwise measure downhole borehole fluid. In logging-while-drilling, however, these types of measurements are not readily available. Also, the types of computed differential caliper or computed caliper presently known can be improved upon.
It is among the objects of the present invention to provide an improved well logging technique and apparatus, that can be used in logging-while-drilling, for determining downhole borehole fluid resistivity, borehole diameter, and borehole corrected formation resistivity of formations surrounding an earth borehole, and for generating logs thereof.